1. Field of the Invention
The present invention relates to a laser annealing method for a semiconductor thin film and a thin film semiconductor device. More specifically, the present invention relates to a laser annealing method for a semiconductor thin film used in a process of fabricating thin film transistors (TFTs) for liquid crystal displays (LCDs), image sensors, SRAMs, and the like, and a thin film semiconductor device fabricated by use of the laser annealing method.
2. Description of the Related Art
In recent years, TFTs have been actively applied to LCDs, image sensors, SRAMs, and the like. In many cases, a number of TFTs are integrated on one substrate. In order to improve the packing density of integrated TFTs, the size of the TFTs should be reduced. For the application of TFTs to an LCD, in particular, it is desirable that a driving circuit for a liquid crystal display panel of the LCD is formed of TFTs arranged on a glass substrate of the panel. Such TFTs for a driving circuit are required to operate at a higher speed than that for TFTs for switching arranged in the display portion. In order to obtain TFTs operating at high speed, a semiconductor thin film having a higher carrier mobility (field effect mobility) is required.
There is known a technique for increasing the carrier mobility of a polysilicon TFT by annealing a semiconductor thin film at a high temperature of 600.degree. C. or more, for example. High-temperature annealing, however, may thermally damage a substrate. Accordingly, such high-temperature annealing is not possible when an inexpensive glass substrate having a low distortion point is used. Non-alkaline borosilicate glass having a distortion point of 650.degree. C. or less (for example, Corning No. 7059) is less expensive than silica glass. For this reason, a technique for forming a semiconductor thin film having a high carrier mobility on such a glass substrate is strongly desired.
A laser annealing method using an excimer laser makes it possible to crystallize amorphous silicon (Si) while the temperature of a substrate is kept as low as about 600.degree. C. or less, so as to form a polysilicon layer having a large grain size. Since thermal damage received by the substrate is small, this laser annealing method is a promising method.
Hereinbelow, a conventional annealing method for obtaining a polysilicon thin film having a large grain size from a non-singlecrystalline Si thin film will be described. In this method, overlap irradiation of the non-singlecrystalline Si thin film with an excimer laser beam is performed. The "overlap irradiation" as used herein refers to a type of irradiation where a region of the non-singlecrystalline Si thin film melted/solidified by the Nth pulse irradiation partially overlaps a region thereof melted/solidified by the (N+1)th pulse irradiation (N is a natural number).
FIG. 3 schematically shows a laser annealing method for a semiconductor thin film adopting the overlap irradiation. Referring to FIG. 3, the reference numerals 3a to 3d denote regions of a non-singlecrystalline Si thin film 4 on a substrate 5, of which crystallinity has changed due to the irradiation with a laser beam 1.
As shown in FIG. 3, the non-singlecrystalline Si thin film 4 on the substrate 5 is irradiated with the laser beam 1 by pulsing. In general, the pulse duration is in the range of 20 to 60 nanoseconds, and the pulse interval is in the range of 3 to 30 milliseconds.
The non-singlecrystalline Si in the region irradiated with the laser beam 1 is first melted and then solidified, thereby changing the crystallinity. More specifically, the silicon shifts from the non-singlecrystalline state to a crystalline state. The "crystalline state" as used herein refers to the state where a plurality of grains each recognized as singlecrystal are formed (generally, called a polycrystalline state). In the crystalline state, the carrier mobility is high when the number of crystal defects is small and when grains are large and thus the area of grain boundaries is small, thereby exhibiting good crystallinity.
By the irradiation with the laser beam 1, the region 3a having a crystallinity different from the surrounding regions is formed in the Si thin film. Then, the relative positioning between the substrate 5 and the laser beam 1 is changed so that the next region to be irradiated with the laser beam 1 partially overlaps the region 3a previously irradiated with the laser beam 1. The relative positioning may be changed by shifting the position of the laser beam 1 or the substrate 5.
Thereafter, the Si thin film is again irradiated with the laser beam 1, so as to form the region 3b having a crystallinity different from the surrounding regions in the Si thin film 4. The same procedure is repeated so as to sequentially form the regions 3c and 3d each having a crystallinity different from the surrounding regions.
Thus, a series of pulses of irradiation are performed by shifting the position of the laser beam 1 with regard to the substrate 5 in a first direction as shown in FIG. 3. Then, the position of the laser beam 1 is shifted in a second direction perpendicular to the first direction. This shift is performed so that the next irradiation region partially overlaps the previously irradiated region. Thereafter, a series of pulses of irradiation is repeated by shifting the position of the laser beam 1 in a direction opposite to the first direction. Alternatively, the position of the laser beam 1 may be returned to a position which is located below the position where the first series of pulses of irradiation started, and then a series of pulses of irradiation may be repeated by shifting the position of the laser beam 1 in the first direction. By repeating such irradiation, the entire surface of the substrate 5 is finally covered with regions irradiated with the laser beam 1.
In general, the irradiation intensity required for the laser annealing of an Si thin film with an excimer laser beam is in the order of several hundreds mJ/cm.sup.2.
FIG. 4A illustrates a section of a conventionally used laser beam and the intensity distribution thereof. FIG. 4B illustrates a conventional overlap irradiation method (beam scanning method).
Referring to FIG. 4A, the reference numerals 1a and 1b denote a flat portion and an edge portion of the laser beam 1, respectively. Referring to FIG. 4B, the reference numerals 2a and 2b denote a region irradiated with the edge portion 1b of the laser beam 1, and a region overlap-irradiated with the edge portion 1b for four consecutive times, respectively. The reference numeral 3 denotes a region of which crystallinity has changed by the irradiation with the laser beam 1.
Original laser light emitted from a laser light source such as an excimer laser has a substantially oval section (minor axis: 10-20 mm, major axis: 30-50 mm). The intensity of laser light emitted from the laser light source exhibits a proximate Gaussian distribution across the section thereof. Such laser light is optically split into a plurality of beam elements, and the split beam elements are re-synthesized so as to overlap one another. Thus, the laser beam 1 as shown in FIG. 4A (beam shaping) is obtained. As shown in FIG. 4A, the section of the laser beam 1 is substantially square (from about 5 mm.times.about 5 mm to about 10 mm.times.about 10 mm). The intensity across the section of the laser beam 1 exhibits substantially trapezoidal distribution including a flat region (the flat portion 1a). Even though a plurality of beam elements are re-synthesized so as to overlap one another, a region having a completely uniform intensity is not obtainable. It is possible, however, to form the laser beam 1 having a region (with a size of 8 mm.times.8 mm, for example) where the variation in the intensity is within the order of .+-.10%. The width of the edge portion 1a of the laser beam 1 is generally in the range of 1-2 mm.
As shown in FIG. 4B, the conventional "overlap irradiation" is performed by shifting the laser beam 1 in a direction parallel to a straight-line portion of the outline of the section of the laser beam 1. In the case shown in FIG. 4B, when the size of the section of the laser beam 1 is L mm.times.L mm, the position of the laser beam 1 shifts by L/4 mm during the interval between the Nth irradiation and the (N+1)th irradiation. This case is expressed herein as "the overlap ratio is three-fourths (75%)". In this conventional case shown in FIG. 4B, the region 2b included in the region irradiated with the edge portion 1b of the laser beam 1, which extends along the shift direction of the laser beam 1 is irradiated with the edge portion 1b of the laser beam 1 for four times. This number of irradiation times depends on the "overlap ratio".
On the other hand, the region 2a extending perpendicular to the shift direction of the laser beam 1 is irradiated with the edge portion 1b of the laser beam 1 only once during the scanning shown in FIG. 4B. After the initial irradiation with the edge portion 1b, the region 2a is irradiated with the flat portion 1a of the laser beam 1 for three times.
As described above, the region 2b is irradiated with the edge portion 1b of the laser beam 1 for four consecutive times. This region 2b extends to form strips on the top surface of the substrate 5.
The inventors of the present invention have found that the above conventional method has problems as follows:
(1) The region initially irradiated with the edge portion 1b of the laser beam 1 is lower in the crystallinity than the region initially irradiated with the flat portion 1a of the laser beam 1.
(2) The region 2b initially irradiated with the edge portion 1b of the laser beam 1 and then also irradiated with the edge portion 1b is lower in the crystallinity than the region initially irradiated with the edge portion 1b of the laser beam 1 and then irradiated with the flat portion 1a thereof.
(3) The region initially irradiated with the edge portion 1b of the laser beam 1 for K consecutive times is lower in the crystallinity than the region initially irradiated with the edge portion 1b for (K-1) consecutive times.
(4) The difference in the crystallinity described above in (2) and (3) remains even though the former region is subsequently irradiated with the flat portion 1a of the laser beam 1.
(5) Once the crystallinity of the region 2b subjected to the overlap irradiation with the edge portion 1b of the laser beam i differs from those of other regions, this causes a variation in the device performance depending on the difference in the crystallinity.